A marine offshore platform of the semi-submersible type is often anchored at each corner column by several anchor chains. Some platform designs has four anchor chains at each corner column. The anchor chains—or the part of them currently not in use, usually referred to as the tail ends of the chains—are stored in separate chain lockers that are normally positioned low in the pontoons of the platform, with vertical chain pipes extending within the columns from the chain lockers to a deck fitted with anchor winches. Similar chain lockers and chain pipes are also used on conventional ships, but in a smaller scale and in lesser numbers.
Due to the large volumes of the chain lockers, and since the chain pipes are directly communicating with the chain lockers, it is normal practice to place the top mouth portions of the chain pipes above as many pre-calculated damage waterlines as possible for stability reasons. To insure the watertight integrity of the vessel as per governing maritime regulatory requirements, the system is dimensioned to cope with serious damage scenarios in which the top mouth portions are submerged. Hence, the top mouth portions of the chain pipes must either be sealed from water intrusion while the anchor chains are stationary, or both the chain lockers and the chain pipes will have to be constantly filled with water in order to avoid an unacceptable notch in the stability curve of the vessel. Since constant water fill will substantially reduce valuable payload of the vessel, various devices and methods for sealing the chain pipes are known.
A problem involved with sealing between the chain pipe and the anchor chain is that the chain links normally have a very coarse outer surface due to corrosion and biological marine fouling, which makes it difficult to obtain an effective sealing contact. The oldest, and perhaps the most drastic solution to this problem is to temporarily mould the top mouth portion of the chain pipe with concrete. The concrete sealing will then be manually hacked up when the anchor chain is to be moved again. This is of course an awkward method which is obviously unsuited for repeated operation of the anchor chains.
Another known solution to the problem is to place two opposing, horizontally slidable cover lids at the mouth of the chain pipe. The slidable cover lids engage the chain link in a “guillotine”-fashion into a closed position in which the cover lids abut each other and seal the chain pipe from water intrusion by sealingly engaging a chain link of the chain. To accommodate for the chain link, the cover lids are each provided with two semicircular recesses shaped in conformity with the contacting outer periphery of a chain link. This solution suffers, however, from an inferior sealing effect around the periphery of the chain link in all but the two opposing—but mutually aligned—sliding directions of the slidable cover lids. This problem is further accentuated by the coarseness of the outer surface of the chain links. Another drawback with this solution is that the slidable cover lids occupy much sideways space in a cramped environment when in an open, retracted position due to protruding rear ends of the lids.